Question: The scale on a map is 5cm : 5km. If the distance between two cities is 10km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 5cm on the map corresponds to an actual distance of 5km. An actual distance of 10km is the same as 2 $\cdot$ 5km. The distance between the two cities on the map is 2 $\cdot$ 5cm, or 10cm.